Thin coating manufactured by vapor deposition of solid oligomers

ABSTRACT

A monomer is selected to produce a polymeric film having desirable characteristics for a particular application. The monomer is polymerized under controlled conditions to produce a solid oligomer having those characteristics at a molecular weight suitable for evaporation under vacuum at a temperature lower than its thermal decomposition temperature. The process of polymerization to produce the oligomer is carried out under conditions that yield a finite molecular-chain length with no residual reactive groups. The solid oligomer so produced is liquefied and extruded as a film onto a revolving drum in the evaporation section of a conventional vapor deposition chamber, and it is then cryocondensed on a cold substrate to form a solid thin film having the same desirable characteristic selected in the solid oligomer constituting the starting material. As a result of the initial complete reaction to produce the oligomer, the thin-film product does not contain unreacted groups and all attendant disadvantages are avoided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is related in general to a process of manufacture of thinpolymer films by vacuum vapor deposition. In particular, it pertains tothe vacuum evaporation and deposition of solid oligomers to form a solidfilm on a substrate directly without subsequent polymerization or curingreaction.

2. Description of the Related Art

Thin metallic and polymeric films are used on various substrates, suchas paper, fabrics, plastic and metal foils, to add or promote desirableproperties for particular applications. For example, foils used topreserve food need to have very low permeability to oxygen; the exteriorsurface of packaging material has to be capable of accepting inks forprinting purposes; and packaging materials for electronic products alsorequire a limited amount of conductivity to dissipate electrostaticcharges. Similarly, metallic pigment flakes are produced by depositing athin layer of metal on a web from which the metal is later separated andcrushed; therefore, it is important that the web material be suitablefor the separation step. To these ends, it is desirable and sometimesnecessary to modify the physical properties of polymeric films toimprove their suitability for the intended purpose. Preferably, though,the films are directly formed with a composition and molecular structurecharacterized by the desired properties.

Thin films of metals and polymers are created by deposition ontoappropriate substrates by a variety of known processes, most notablythrough film formation by wet chemistry or vapor deposition. Chemicalprocesses produce soluble thermoplastic as well as insoluble thermosetpolymers and involve the use of solvents; thus, film formation isachieved through solvent diffusion and evaporation. As a result, theseprocesses require relatively long residence times and the undesirablestep of handling solvents.

Vapor deposition processes involve the evaporation of a liquid monomerin a vacuum chamber, its deposition on a cold substrate (referred to inthe art as “cryocondensation”), and the subsequent polymerization byexposure to electron beam or ultraviolet radiation. As illustratedschematically in FIG. 1, the liquid monomer from a supply reservoir 12is fed through a capillary tube 14 and an atomizer 16 into the heatedevaporator section 18 of a vacuum deposition chamber 10 where it flashvaporizes under vacuum. The resulting monomer vapor is then passed intoa condensation section 20 of the unit where it condenses and forms athin liquid film upon contact with the cold surface of an appropriatesubstrate, typically a rotating drum 22. A metal vaporization unit 24may also be used to deposit in line a thin metal layer on the drum 22for multilayer deposition. The liquid deposited film is then cured byexposure to an electron beam or ultraviolet radiation source 26. Sincethe ultimate objective is the formation of solid films, the initialliquid monomer must be capable of polymerization and contain enoughreactive groups to ensure that a sufficiently large polymeric moleculeresults and yields a solid product.

This conventional approach of utilizing a polymerizable monomer as theraw material for thin-film forming processes has been followed over theyears because it is not possible to vaporize the final polymeric productunder the range of operating conditions of a commercially viable vapordeposition chamber (typically, 10⁻³ to 10⁻⁶ torr and 70° C.-350° C.).Thus, the practice in the industry has been to identify or developpolymers having specific characteristics deemed advantageous for aparticular film application. A solid thin film of the polymer is thenformed on a target substrate by evaporating the corresponding liquidmonomer, cryocondensing it as a monomer in liquid form and polymerizingor cross-linking it to reach the required molecular weight to ensure itssolidification. Many variations of this basic approach have beendeveloped for particular applications, but all prior-art vacuumdeposition processes involve the formation of a solid film bypolymerization of a liquid monomer evaporated under vacuum oratmospheric conditions and recondensed on a cold surface to obtain thedesired film characteristics.

This approach has several inherent constraints that often producedisadvantages. Most importantly, the selected monomers have to includereactive moieties, such as acrylate and vinyl groups, in order to enablepolymerization and produce a solid, usable film after condensation.Thus, the resulting polymeric film necessarily contains reactive groupsthat give the film reactive characteristics that may be undesirable forparticular uses. For example, the oxygen atoms in acrylate groups givesthe film adhesion properties that prevent it from being used as arelease coating.

Because thin-film forming by vapor deposition is commerciallypreferable, considerable research has been conducted to developprocesses for improving the properties of thin films obtained bypolymerization of vacuum deposited monomers. For example, U.S. Pat. No.5,681,615 discloses a method for incorporating a salt of desirablecharacteristics into the polymeric film. The salt is dissolved in theinitial liquid monomer, which is then vaporized at a temperature belowthe decomposition and polymerization temperatures of the compositematerial and cryocondensed according to conventional practice.

In U.S. Pat. No. 5,902,641, a further improvement is introduced byvaporizing a two-phase mixture of a liquid monomer and insoluble solidparticles, thereby producing a homogeneous vapor mixture of the twocomponents. The mixture is then cryocondensed and the monomer ispolymerized or cross-linked according to standard procedures to yield athin film that contains the solid component in uniform, homogeneousdistribution. Thus, the process enables the manufacture, for example, ofdoped polymeric films for the electronic industry.

The main drawback of all prior-art vapor-deposition techniques is thefact that they require polymerization or cross-linking of theliquid-film monomer formed by vacuum evaporation and cryocondensation.Thus, the presence of residual reactive groups in the final product isunavoidable.

Therefore, there is still a need for an approach to thin-film polymermanufacture that makes it possible to produce films without the inherentdisadvantages that result from polymerization and/or cross-linking. Thisinvention is directed at a process for achieving this objective byjudiciously selecting appropriate oligomers as the starting material forthe manufacture of thin films.

BRIEF SUMMARY OF THE INVENTION

One primary objective of this invention is a method for producingpolymeric thin coatings that do not contain undesirable reactive groupsin a solvent-free, environmentally safe process.

Another important objective is a process that produces a thermoplastic,soluble and/or fusible, and possibly recyclable coating.

Another objective is a process that can be implemented at a highproduction rate.

Still another objective is a process that can be implemented to coatlarge surface areas.

Another goal is a process that produces a highly uniform, defect-freethin film.

Another objective is a process that makes it possible to select theproperties of the vapor-deposition raw material to tailor the desiredproperties of the thin-film product.

Specifically, a goal of the invention is a process for manufacturingthin films suitable as release coatings.

Another specific goal is a process for manufacturing thin films suitablefor packaging of food and electronic products.

Another objective is a procedure that can be implemented utilizingmodified prior-art vapor deposition technology.

Still another goal of the invention is an apparatus suitable forcarrying out a process for producing polymeric thin films that do notcontain undesirable reactive groups.

Similarly, a specific objective is an apparatus for manufacturing thinfilms that are suitable as release coatings.

Another goal of the invention is a device for introducing a solid feedinto a vapor-deposition chamber such that it can be flash evaporated atcontrolled feed rates under conventional temperature and pressureconditions.

A final objective is a procedure that can be implemented easily andeconomically according to the above stated criteria.

Therefore, according to these and other objectives, the presentinvention consists of selecting a monomer that is known to produce apolymeric film having desirable characteristics for a particularapplication. The monomer is polymerized under controlled conditions toproduce a solid oligomer having those characteristics at a molecularweight suitable for evaporation under vacuum at a temperature lower thanits thermal decomposition temperature. The process of polymerization toproduce the oligomer is carried out under conditions that yield a finitemolecular-chain length with no residual reactive groups. The solidoligomer so produced is liquefied and extruded as a film onto arevolving drum in the evaporation section of a conventional vapordeposition chamber, and it is then cryocondensed on a cold substrate toform a solid thin film having the same desirable characteristic selectedin the solid oligomer constituting the starting material. As a result ofthe initial complete reaction to produce the oligomer, the thin-filmproduct does not contain unreacted groups and all attendantdisadvantages are avoided.

According to another aspect, the invention relates to a modifiedextruder, feed nozzle and rotating hot drum for heating and vaporizing asolid oligomer under vacuum conditions. The oligomer is preferablygranulated and fed into the extruder wherein the solid particles areheated and gradually melted as they advance toward the extruder'soutlet. The nozzle shapes the liquified material into a thin film thatis deposited on the revolving hot drum in a conventional vacuumevaporator, whereby the liquid film flashes into a homogeneous vaporthat is subsequently cryocondensed to form a thin film of the samemolecular weight of the starting material.

Various other purposes and advantages of the invention will become clearfrom its description in the specification that follows and from thenovel features particularly pointed out in the appended claims.Therefore, to the accomplishment of the objectives described above, thisinvention consists of the features hereinafter illustrated in thedrawings, fully described in the detailed description of the preferredembodiment and particularly pointed out in the claims. However, suchdrawings and description disclose but one of the various ways in whichthe invention may be practiced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a prior-art vacuum vapordeposition and radiation curing unit.

FIG. 2 is a schematic representation of a modified vacuum vapordeposition unit according to the invention.

FIG. 3 is a partial schematic representation of the front portion of thenozzle of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

One aspect of the present invention lies in the recognition thatoligomers and polymers derived from a given monomer often have the samephysical and chemical characteristics and that, therefore, they can beused interchangeably in the manufacture of thin-film coatings. Based onthis initial realization, the heart of the invention lies in the idea ofevaporating a solid oligomer selected to have a vaporization temperaturebelow its thermal decomposition temperature under the vacuum conditionsattainable with conventional vapor deposition equipment. Thus, theresulting vapor can be cryocondensed directly into a solid thin filmcharacterized by the same properties of interest without the need forfurther polymerization, as required in all prior-art techniques.

As used herein, the term oligomer is intended to refer not only tomolecular chains normally designated as such in the art (typicallycontaining between two and ten monomer molecules) but also tolow-molecular weight polymers; specifically, oligomer is meant toencompass any polymerized molecule having a molecular weightsufficiently low to permit its vaporization under vacuum at atemperature lower than its temperature of thermal decomposition. Withcurrent vacuum technology, such maximum molecular weight isapproximately 5,000, the precise molecular weight depending on thespecific monomer used, but it is understood that greater molecularweights would become available for inclusion in the practice of theinvention if greater vacuum conditions were obtained. Therefore, theinvention is not to be limited to polymeric chains with molecular weightless than about 5,000, but is intended to include any polymericmolecule, herein defined as oligomeric, that is solid at the temperatureand pressure of its intended use and is capable of vaporization at atemperature lower than the temperature at which it decomposes orotherwise degrades. Thus, the term oligomer alone, as here defined, isused for convenience to describe the invention.

A solid oligomer for the purposes of the invention is defined as a lowmolecular-weight polymeric composition that is solid at the temperatureand pressure conditions of its intended use. As well understood in theart, the term cryocondensation refers to a phase-change process from gasto liquid obtained upon contact with a surface having a temperaturelower than the dew point of the gas at a given operating pressure. Theterm thin film is intended to encompass any layer of material with athickness in the order of microns, the thickness typically consisting ofpolymeric films produced either by vapor deposition or by wet-chemistryprocesses. Finally, the term degradation is used interchangeably withthe term decomposition.

One aspect of the invention involves the polymerization of a selectedmonomer under controlled conditions to produce a solid oligomer havingthe properties targeted for the thin-film product to be manufactured byvapor deposition. The oligomer is selectively produced with a molecularweight sufficient to provide the desired film characteristics as a solidat the temperature of application. In addition, the oligomer must have avaporization temperature that is below its decomposition temperature atthe operating pressures of conventional vapor deposition chambers (10⁻³to 10⁻⁶ torr), and that is also within the temperature operating rangeof such equipment (70° C.-350° C.). The oligomer so produced isseparated from unreacted monomer molecules to eliminate substantiallyall reactive groups from the raw material used to form the target thinfilm, as intended in order to obtain an unreactive product.

According to another aspect of the invention, the oligomer material isgranulated or otherwise reduced to particles and fed into a hopper forprocessing in a modified conventional vacuum vapor deposition unit 30,as illustrated schematically in FIG. 2. The solid oligomer particles arefed from the hopper 32 into a heated extruder 34 that liquefies thematerial and forces it out of a nozzle 36 as a plurality of thin liquidfilm strips suitable for flash evaporation. As illustrated in FIG. 3,each slit opening 37 in the nozzle 36 is about 1-2 mm wide and 4-5 mmlong.

The nozzle 36 extrudes the liquid film continuously onto a heatedrotating drum 38 under vacuum which causes its evaporation upon contact.This novel assembly of parts constitutes the evaporator of the vapordeposition unit 40 of the invention.

The resulting vaporized oligomeric molecule is then passed through aslit 42 to reach the cryocondensation section in the vapor depositionunit. Upon contact with a rotating cold drum 44 (typically kept at −20°C. to 30° C.), the vapor condenses and forms a uniform, homogeneous thinfilm that quickly solidifies into the final product. As in prior-artvapor deposition units, the thin film may be deposited over the drum 44to produce a multilayer product or over a web substrate in contact withthe drum to produce a film coating. No further polymerization isrequired to solidify the film.

In order to ensure the uniform deposition of the vaporized oligomer overthe condensation drum 44, it is critical that the feed rate of oligomerto the evaporator be controlled precisely. The use of an extruder, whichcan be run at a well defined and controlled extrusion rate, was found tobe ideal for feeding the hot drum in the evaporator section of the vapordeposition unit of the invention. At the same time, the use of a slitnozzle that produces a very thin liquid film of oligomer makes itpossible to approximate the very large surface area created byatomization, thereby enabling the immediate vaporization of the liquidoligomer upon contact with a hot surface. It is noted that vaporizationof the liquified oligomer by regulating its flow through a solenoidvalve and passing it through an atomizer, as done in the prior art,would not afford the degree of feed control required for the process ofthe invention because oligomers are too viscous. Thus, the use of anextruder is ideal for this application.

The following examples illustrate the invention.

EXAMPLE 1

This example illustrates the manufacture of a solvent-free, flashevaporated and vacuum deposited thermoplastic (soluble and fusible)release coating. The object is the preparation of a release coatingusable for the manufacture of metal flakes utilized in pigmentproduction. According to the prior art, the metal flakes are produced bydepositing a metal film over a polymeric substrate coated with apolymeric film, and then by separating the metal film from the substrateand crushing it to yield flakes. Existing technology is based onrelatively low-rate solvent-based processes for releasing the metal filmfrom the polymeric substrate.

The materials selected for thies application of the invention arehydrocarbon polymers (e.g., polyethylene and polystyrene) which areknown to exhibit very poor adhesion to substrate films and to other topcoatings (metal, ceramic). The metal flakes are produced by depositing ametal film over an oligomeric release coating formed by vapor depositionover a substrate according to the invention. The release process can bemechanical peel off, melting of the release coating, or dissolving ofthe release coating.

First Run: A polyethylene oligomer of molecular weight 4000, availablefrom Aldrich Chemicals Company of Milwaukee, Wis., was melted at 130° C.and extruded into a 10⁻⁵- to 10⁻⁶ torr chamber to be spread on a hotrotating drum (at about 300° C.) and evaporated (see FIG. 2). The formedvapor, driven by vacuum, was passed through a slit nozzle from theevaporating area to a deposition chamber and deposited as a solidcoating (0.5 to 1.0 micron thick) on a cold polyester web (in intimatecontact with a drum kept at about 0° C.). Aluminum film was depositedin-line on top of the polyethylene-oligomer release coating according toconventional vapor deposition. The web was running at 200 ft/min.Aluminum metal flakes were released from the substrate film by crushingit, and then either melting (at about 100° C.) or dissolving (intoluene) the release layer.

Second Run: A poly(α-methylstyrene) oligomer of molecular weight 1300,available from Aldrich Chemicals, was melted at 150° C. and extrudedinto a 10⁻⁵-10⁻⁶ torr vacuum chamber to be spread on a hot rotating drum(at about 300° C.) and evaporated. The formed vapor, driven by vacuum,was deposited as a solid coating (0.3 to 0.8 micron thick) on a coldpolyester web (at about 0° C.). An aluminum film was deposited in-lineon top of the polyethylene-oligomer release coating. The web was runningat 200 ft/min. The metal flakes were released from the substrate film bycrushing, and then melting (at about 100° C.) or dissolving (in toluene)the release layer. Similar products were produced successfully withpoly(α-methylstyrene) oligomers of molecular weight up to 2000.

EXAMPLE 2

This example illustrates the manufacture of multilayer strap releaselayers (existing technology consists of crosslinked insoluble andinfusible release layers). The object is again the preparation of arelease coating usable for the manufacture of metal flakes utilized inpigment production.

A multilayer sequential metal-release coating strap can be produced bythe technique of the invention. The thin metal layers can be releasedand crushed to pigments by melting or dissolving the thermoplasticrelease layers which separate the metal layers.

Run 1: A polyethylene oligomers raw material of molecular weight 4000,available from Aldrich Chemicals, was melted at about 130° C. andextruded in 10⁻⁵-10⁻⁶ torr vacuum to be spread on a hot rotating drum atabout 300° C. and evaporated. The formed vapor was deposited as a solidcoating (0.3 to 0.5 micron thick) on a rotating cold drum (kept at about0° C.). Aluminum film was deposited in-line on top of the polyethylenefilm. The drum was rotating at 200 ft/min. A multilayer sequential(about 5000 layers) metal-release coating strap was formed. The metalflakes were then produced by extracting the polyethylene release layerby either melting it (at about 100° C.) or dissolving it (in toluene).

Run 2: Poly(α-methylstyrene) oligomers of molecular weight 1300,available from Aldrich Chemicals, were melted at about 150° C. andextruded in 10⁻⁵-10⁻⁶ torr vacuum to be spread on a hot rotating drum(kept at about 300° C.) and evaporated. The formed vapor was depositedas a solid coating (0.3 to 0.5 micron thick) on a rotating cold drum(kept at about 0° C.). Aluminum film was deposited in-line on top of thepolyethylene film. The drum was rotating at 200 ft/min. A multilayersequential (about 5000 layers) metal-release coating strap was formed.The metal flakes were then produced by extracting the polyethylenerelease layer by either melting it (at about 100° C.) or dissolving it(in toluene). Similar products were produced successfully withpoly(α-methylstyrene) oligomers of molecular weight up to 2000.

EXAMPLE 3

This example illustrates the manufacture of electrically conductivecoatings (existing technology is a solvent-based, slow coating process).Polyaniline oligomers can be prepared according to syntheses techniquesthat offer absolute control over the molecular weight and the end groupof the polymer. These techniques are well known in the art. See, forexample, E. Rebourt et al., “Polyanaline oligomers; Synthesis andCharacterization,” Synthetic Metals 84 (1997) 65-66; and K.Matyjaszewski et al., “Synthesis of Well-Defined Polyacrylonitrile byAtom Transfer Radical Polymerization,” Macromolecules, Vol. 30, No. 20,1997. The degree of polymerization can thus be optimized to produceconducting oligomers that meet the requirements for vacuum deposition.The same concept can be applied to polypyrrole and polythiophene,leading to high speed, large scale commercial production for a varietyof electroactive conducting polymer coatings useful in a multitude ofapplications (electrostatic dissipation, corrosion resistance, andsensor manufacturing).

Run 1: Polyaniline oligomers, average molecular weight 550-850, weresynthesized and melted/extruded at about 150° C. in a 10⁻⁵-10⁻⁶ torrvacuum for spreading over a hot rotating drum (at about 350° C.) andevaporated according to the invention. The formed vapor, driven byvacuum, was deposited as a solid coating (1.0 to 3.0 micron thick) on acold polyester web at about 0° C. The web was running at 100 ft/min. Thepolyaniline film was doped with 0.1 molar hydrochloric acid solution,dried, and characterized for various properties. Similar runs wereperformed successfully with polyaniline oligomers of molecular weight upto 1000.

Run 2: Polyaniline oligomers, average molecular weight 550-850, weresynthesized and mixed with an equivalent amount of tetracyanoethylene asa dopant, then melted/extruded at about 150° C. in a 10⁻⁵-10⁻⁶ torrvacuum, and spread on a hot rotating drum (at about 350° C.) andevaporated. The formed vapor, driven by vacuum, was deposited as a solidcoating (1.0 to 3.0 micron thick) on a cold polyester web at about 0° C.The web was running at 100 ft/min. The doped polyaniline film wascharacterized for various properties.

EXAMPLE 4 Profetic

This example illustrates the manufacture of high-barrier coatings forpackaging materials (existing technology consists of solvent basedcoatings). The object is the preparation of a coating with very lowpermeability to oxygen and vapors.

It is expected that low molecular weight polyacrylonitrile orpolyvinylidene chloride (up to 5000 Mw), having the necessary propertiesfor practicing the invention, can be flash evaporated and vacuumdeposited on various film substrates to produce barrier coatings forpackaging materials.

These examples demonstrate the feasibility of manufacturing thinpolymeric coatings by vapor deposition utilizing solid raw material. Theresulting process possesses the advantages of being solvent-free,environmentally safe, and high rate. Large areas of coating (up to 1000ft/min) can be applied at low cost. The resulting products are in theform of highly uniform, defect-free coatings.

Various changes in the details, steps and components that have beendescribed may be made by those skilled in the art within the principlesand scope of the invention herein illustrated and defined in theappended claims. Therefore, while the present invention has been shownand described herein in what is believed to be the most practical andpreferred embodiments, it is recognized that departures can be madetherefrom within the scope of the invention, which is not to be limitedto the details disclosed herein but is to be accorded the full scope ofthe claims so as to embrace any and all equivalent processes andproducts.

We claim:
 1. A process for forming a solid thin-film layer of oligomericmaterial in a vapor deposition unit operating at a predeterminedpressure, a predetermined evaporator temperature, and a predeterminedcondenser temperature, the process comprising the following steps: (a)polymerizing a monomer to obtain a solid oligomer thereof having avaporization temperature at said predetermined pressure that does notexceed said predetermined evaporator temperature, and having adecomposition temperature at said predetermined pressure that exceedssaid predetermined evaporator temperature; (b) flash evaporating thesolid oligomer in said vapor deposition unit to produce a vapor at saidpredetermined pressure and evaporator temperature; and (c)cryocondensing the vapor to produce a solid oligomer in thin-film formin the absence of irradiation during the flash evaporating andcryocondensing steps.
 2. The process of claim 1, wherein said flashevaporating step is carried out by liquefying the solid oligomer anddepositing a thin film thereof on a surface maintained at saidpredetermined evaporator temperature.
 3. The process of claim 1, whereinsaid predetermined pressure is about 10⁻³ to 10⁻⁶ torr.
 4. The processof claim 1, wherein said predetermined evaporator temperature is about70° C.-350° C.
 5. The process of claim 1, wherein said predeterminedcondenser temperature is about −20° C. to 30° C.
 6. The process of claim1, wherein said predetermined pressure is about 10⁻³ to 10⁻⁶ torr, saidpredetermined evaporator temperature is about 70° C.-350° C., and saidpredetermined condenser temperature is about −20° C. to 30° C.
 7. Theprocess of claim 1, wherein said monomer is ethylene used to prepare arelease coating and said oligomer has a molecular weight not to exceedabout
 4000. 8. The process of claim 1, wherein said monomer isα-methylstyrene used to prepare a release coating and said oligomer hasa molecular weight not to exceed about
 2000. 9. The process of claim 1,wherein said monomer is aniline used to prepare an electricallyconductive coating and said oligomer has a molecular weight not toexceed about
 1000. 10. The process of claim 1, wherein said monomer isacrylonitrile used to prepare a high-barrier coating and said oligomerhas a molecular weight not to exceed about
 5000. 11. The process ofclaim 1, wherein said monomer is vinylidene chloride used to prepare ahigh-barrier coating and said oligomer has a molecular weight not toexceed about 5000.